in the Debian's BTS I've found this old bug:
What do you think about adding the support for the MP3 format to the next release of audiofile?
The Debian BTS patch contains the C code for an MP3 decoder, so is subject to license fees in some jurisdictions.
One way to do this, getting round all the licensing hassle, is to look for external programs like lame or mpg123, and get them to decode the file, presumably starting them up on a pipe as a read-only non(-backwards)-seekable stream. Legally speaking, I presume, if the user has those tools installed, that means they are authorised to use them in their jurisdiction. Either way, libaudiofile is not shipping an MP3 codec.